1. Technical Field
The present disclosure generally relates to a surgical apparatus and associated methods for fusing two adjacent bone structures such as vertebrae of the spine using an anterior or posterior interbody approach.
2. Background of the Related Art
The deterioration of a body joint such as an intervertebral disc causes the joint space to undergo degenerative changes including narrowing of the joint space and stiffening of the joint. This degeneration of the joint space may lead to mechanical instability of the joint and become severely painful. When no other alternative treatment suffices to stop the disabling pain the joint may have to be fused together.
The fusion process for intervertebral discs typically requires surgically altering the joint surfaces with removal of the articular cartilage and internal tissues attached to the bone. A mechanical device and/or bone material is inserted into the joint to cause the two formerly moving surfaces to fuse or bridge together via the inserted device or bone. Due to various natural effects, bone fusions grow slowly. As such, the bony union may require a period of several weeks or months of bone ingrowth to have sufficient strength to support normal joint loading. The healing period is of course dependent upon such factors as the patient""s age, the location of the joint, the forces applied to the joint and the rate by which the bony union progresses in the particular patient. A successful fusion demands that the bone structure of the one bony component of the joint grow together with the bone structure of the second bony component of the joint thereby creating a solid union between these two bony components.
All bones are composed of cortical and cancellous portions, the cortical portion being a thin, hard outer shell and the cancellous portion including an internally soft material. It is known that the most successful fusion promoting substance to be inserted between the two joint components is cancellous or soft bone taken as a graft from a donor site within the patient""s body. This soft bone constitutes an autograft and contains growth promoting substances and biochemical materials which accelerate the rate of growth and quality or solidity of the resultant bone fusion. Further, the bone graft material must be supported and stabilized so that it is not subjected to motion or dislocation. During the growth of the bone fusion, a space less than 200 xcexcM between the bone components and the fusion material will inhibit good bone growth. However, a space of this size or larger permits the ingrowth of fibrous tissue causing the resulting fusion to be poor in strength or to fail to fuse altogether. Along the same lines, motion within the fusing joint or between the bone graft particles will also inhibit bone growth and subsequently inhibit a secure attachment of the bone graft particles to the joint""s bony components. In addition, the bone graft material must be brought into contact with a bleeding or vascularized surface of the bone joint to be fused. Since the cancellous inner bone has good intrinsic circulation which is vital to fusion growth, the outer cortical bone must be cut or ground away such that the vascularized cancellous inner bone is exposed and bleeding. It is to this bleeding or vascularized surface that the bone graft is applied.
Proper bone fusion requires that the bone graft material be held firmly in place within the joint space without any excess movement throughout the fusion process. Many methods and devices have been devised to secure the bone graft firmly in place as well as to secure the bony components of the joint in the desired position as the bony fusion slowly develops. Conventional prior art fusion devices are not suitable for the requirements for which the disclosure has been developed. For example, U.S. Pat. No. 4,961,740 to Ray et al. discloses an interbody cage having an internal cavity with an inner surface and an outer surface. A pair of these devices is screwed into parallel round cavities drilled into the adjacent end plates of the vertebral disk bodies. These cavities traverse the end plates of each vertebra penetrating into their cancellous bony vertebral substance. The cavities are then tapped and tight fitting metal cages are screwed into the cavities. The cages hold the bone graft and the vertebral bodies firmly in place. Perforations that face the vertebrae are abundant, up to 70% of the outer surface, but the lateral sides of the cages that face the disc space interposed between the vertebrae are blocked against possible soft tissue ingrowth. Such circular fusion devices must penetrate through the cartilaginous vertebral end plate and into the spongy bone of the vertebral body in order for the bone graft material to grow into the vertebral body and create a solid fusion.
The physical shape, namely the height, of a degenerative vertebral disk is dependent upon its actual state of degeneration. In the less degenerated disc, the diameter of the circular fusion cage must be increased to conform with the disk shape. The maximum diameter of a single cage that can be accepted in a given disc joint is limited by the space between the facet joint or pedicle, laterally, and the posterior disc midline. Thus, there is a limit to which the cage can effectively span the disc in relation to the disk height required and the disk posterior width available. The fusion device of the disclosure allows for an increase in height without a resulting concomitant increase in width.
For successful fusion growth development, the recipient bone surfaces must have the cortical or hard surface portion removed. Beneath this hard surface, the cancellous or soft inner portion of the bone, containing its own circulation will then be exposed to the placement of fusion inducing substances such as cancellous or soft bone from another human (allograft) or from the same patient (autograft). When these fusion inducing substances are first placed within the recipient bone, they have little cohesive strength and therefore are very soft and loosely packed. Therefore, a number of devices and appliances have been developed to hold the bony segments in place under conditions of normal spinal activity and daily stresses. The bone graft material being placed between these segments will slowly reunite the segments. Such devices are not, by themselves, intended to permanently secure immobility of the segments, since bone ingrowth is required to produce the stable fusion.
Dependency on any non-uniting device as the sole stabilizing element may ultimately fail due to the development of mechanical transitions between the bone and the device which will lead to a structural failure of the bone.
Fusion bone material placed between vertebral bodies has been described for some years, but more recently the development of pedicle screw fixation and posterolateral instrumentation has become increasingly popular because of the improvement in percentage fusion rate as compared to the earlier interbody fusion methods. However, the pedicle screw technique has been fraught with a number of problems, particularly related to the patient""s safety. Most recently, interbody fusion methods utilizing a bone container, such as a threaded fusion cage, have become increasingly popular because of the improvement in safety and efficacy over other methods and because of lower incidences of complications.
The interbody fusion method is known to be a more efficient technique as compared to methods where bone material is placed around the outside of the vertebral bodies. The interbody fusion is at the center of motion of the spinal segment and requires the least volume of bone to effect a good bone fusion. Further, the fusion enhancing bone material is nearly surrounded by the cortical and/or cancellous bone of the vertebra which provides good nutrition for the fusion growth. For bone material which is laterally placed, nutrition is usually derived from the under surface of the surrounding muscle which is vascularized during the insertion of the fusion device.
The use of cylindrical interbody fusion devices are simpler and safer to implant than are rectangular bone grafts or fusion enhancing devices. To implant a pair of threaded cylindrical fusion devices by a posterior approach, the disc space is entered via two parallel penetrations, one on either side of the central spinous process. Two holes are then drilled or tapped into the interposed disc space and into the adjacent surfaces of the vertebral bones so as to accommodate the two parallel hollow cages. In the case of implanting a pair of threaded cylindrical fusion devices by an anterior approach, two holes are drilled or tapped in close proximity. Screw threads are then cut into the recipient bone bed. The screw threads penetrate into each of the vertebral bodies by a distance of about 3 mm which is sufficient to permit direct contact with vascularized cancellous portion of the vertebrae.
The implantation of a pair of fusion devices is important for stability of the joint space but the method for inserting them must abide by certain anatomical limitations. For example, a singular implant of large diameter of more than 18 to 20 mm cannot be implanted by a posterior approach since the nerves cannot be retracted far enough from either side of the midline to permit such a large device to be safely inserted. The excessive nerve retraction required could readily lead to a nerve stretch injury with damage to nerve function resulting in postoperative severe pain or partial paralysis. Although a range of diameters of the inserts must be available to accommodate disc spaces of different height, fortunately, it has been found that only two different lengths (21 mm and 26 mm) of the implants are needed to accommodate the normal range of vertebral sizes.
The height of the disc space determines the diameter of the insert to be implanted. The distance between the pedicles, from side-to-side across the disc space of the vertebral body ranges from about 30 mm to 45 mm in different sized patients. This distance limits the transverse space available to one or more implants. However, the entire width between the pedicles cannot be used since the vertebrae are oval shaped and the corners of the implants cannot extend outside the vertebral body oval. To do so would otherwise damage or endanger important nerves or major blood vessels that closely approximate the vertebrae. Thus, the combined diameters of a pair of implant devices cannot be wider than about 6 mm less than the overall vertebral body width along the disc level. Therefore, the available practical width usable for a pair of cylindrical implants ranges from about 24 mm to 39 mm. Since each cylindrical implant device must penetrate about 3 mm into each vertebral body so as to contact the cancellous portion of the bone, a disc height equalling or exceeding about 12 mm would require each cylindrical device to be about 18 mm to 20 mm in diameter. However, a pair of such sized devices cannot physically be accepted into a side-to-side arrangement width of the intervertebral disc space. As such, a transversely narrow vertebral segment having a high disc degradation space cannot accommodate two parallel cylindrical implants. Clearly, an improved implant having the ability to increase vertical height without the associated increase in width is needed in the art.
In order for an interbody fusion device to be stable once implanted within the disc space, it is necessary that the device and its implantation technique stretch the anulus fibrosus, the ligamentous band surrounding the outer portion of the disc. The effective elastic recoil effect of this tough ligament plus the patient""s body weight and paravertebral muscle tone, collectively, apply considerable force from both vertebral bodies through the implanted fusion implant, thereby stabilizing the device within the intervertebral space. Further, a pair of such cylindrical implants parallelly placed into the disc space provides important segmental stability as the bone fusion grows. This stability must withstand normal lateral flexion-extension and torsional forces applied to the segment. A singular cylindrical implant may provide considerable torsional and flexion-extension stability when implanted parallel to the front-back axis of the disc space, but would not provide adequate stability in lateral side-to-side bending as the segment would hinge over the implant.
The collapse of an implanted cylinder is prevented by two mechanisms, first, the arc of the cage pressing into the vertebral bone includes a distinct compression strength. Secondly, the greater diameter of the implanted cylindrical fusion device is wider than the hole bored into the two vertebrae, that is, the maximum width of the device lies in the disc space inside the vertebral end plates. Therefore, for such a device to further penetrate into either end plate it must stretch the end plate cortical bone. This portion of the cortical bone is the strongest portion of the vertebral body and resists such stretching forces. In actual clinical applications, the implant cages have penetrated into the vertebral bodies by less than 1 mm. The intactness of the cortical edge of the end plate is therefore important to prevention of the collapse of the vertebrae around the implants. A substantial loss in disc space height would be detrimental to the posterior ancillary structures of the spinal segment including the anulus, facet joints and ligaments.
A spherical, expandable spinal implant is disclosed in U.S. Pat. No. 5,059,193 to Kuslich. The Kuslich implant includes deformable ribs which may be expanded outwardly once installed inside the prepared disc space. As a spherical implant, however, it is inherently unstable as was ball bearing type implants disclosed by U. Fernstrom in 1966. The Fernstrom device, intended as an artificial disc, proved to be a non-functional device and most of the several hundred devices implanted had to be later removed.
A spine fusion implant having an oval contour is disclosed in U.S. Pat. Nos. 5,458,638 and 5,489,308 to Kuslich et al. The Kuslich et al. implants include slots along its outer periphery towards the vertebral bodies. The side walls are blocked against invasion of disc material as was described in the literature by Ray. The oval shaped insert requires the drilling of three adjacent holes such that the height is at least twice the width. This concept addressed the same limitations in disc width space versus disc height space as discussed above. The Kuslich et al. implants are not expandable and any potential combination of increased height plus expandability are not disclosed by the Kuslich et al. references.
Furthermore, the Kuslich et al. patents dislose that the semi-cylindrical arcuate ribs are not tapered for the purpose of prevention of expulsion or pullout after insertion into the prepared disc space, but rather to promote ease of insertion without concern for expulsion except as may be provided by the settling of vertebral spongy bone into the slots between the ribs.
The expandable non-threaded spinal fusion device of the disclosure overcomes the difficulties described above and affords other features and advantages heretofore not available.
The device disclosed herein provides a series of resilient supporting arches which act as spacers between the two vertebral bodies, but also permit a simple partial collapse of about 1 mm of soft bone into the spaces between the arches. These arches preferably have parallel slots machined perpendicular to the long access of the implanted device. After insertion of the device, a combination of body weight and muscular contractions applied across the vertebrae and device serve to allow the vertebral bone to descend or sink into the parallel slots of the device. The vertebral bone will descend or sink across the device to a point that will allow fusion promoting substance, i.e. bone material or any of the well known substitutes such as bone morphologic protein, hydroxyapatite or bone growth factor, placed within the slotted arches to be brought into contact with the bone of the vertebral body. Furthermore, the device can be made in a narrow range of sizes since the two halves of the device are placed into a hole bored between the vertebral bodies and then the halves of the device are forced apart to penetrate into the softer bone of the vertebral spongiosa or cancellous bone. Thus, both the width and height of the devices are separately controlled.
The cortical portion of the juxtaposed end plate of the vertebra is cut away by a drilling process thereby forming the hole which will accommodate the two halves of the slotted cage. An insertion tool or spreading device delivers the two halves of the cage inside the hole and then spreads the two halves apart to force the parallel ribs of the cage into the recipient soft bone.
The spreading device elevates and/or separates the two halves of the cage until the outer anulus of the cage becomes abutted tightly against the receiving bone and capable of exerting sufficient counter force to stabilize each of the slotted cages. While being spread apart by the spreading device, notched rod-like spacers of various heights may then be inserted into the lateral stabilizing structures or channels of each cage. Once the notched spacers are inserted, the spreading device is released and removed from within the two halves of the cage. At this time, the recoil force of the outer anulus of the cage will force the lateral portions of each cage against the spacers further stabilizing them.
In addition, the insertion tool is capable of moving either one of the cage halves further out of or further into the drilled holes of the vertebral body in order to compensate for any slippage between the two vertebral bodies which may have occurred as a result of injury or degeneration. Once the two halves of the cage are situated in the drilled holes of the vertebral body, the insertion tool can then be used to correct the slippage and alignment before the notched spacers are placed. After properly aligning the vertebral bodies, the notched spacers are inserted and positioned along the lateral stabilizer channels of the cage. The insertion or spreading tool is then removed allowing the recoil of the outer anulus of the cage to force the ribs of the slotted arches into the bone, thereby stabilizing the now corrected displacement of the vertebral bodies.